Method and apparatus for forming a liquid-forged article

ABSTRACT

A method of forming a liquid-forged article. The method comprises introducing a melt into a die cavity; moving a punch relative to the die cavity such that the melt enters at least one high aspect ratio cavity in the punch and air is released via at least one air vent insert in the punch; and exerting a forming pressure on the melt while the melt solidifies in the high aspect ratio cavity.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus for forming aliquid-forged article and particularly, though not exclusively, toforming a liquid-forged near net-shape article having features with highaspect ratio.

BACKGROUND

Articles having features with high aspect ratio are conventionally madeby methods including machining, extrusion, forging, or casting a metalsuch as aluminium (Al). Machining is a process that produces articleswith high tolerance control and good surface finishing. However, it istime consuming and expensive and results in a large amount of materialwastage, making it unsuitable as a process for mass production.Extrusion and forging are processes that can produce articles with goodstructural integrity. However, traditional extrusion and forging areunable to form articles requiring a high density of features having highaspect ratio, such as heat sinks having fins and pins in close proximityas shown in FIGS. 1 and 2.

Die casting (high and low pressure) is a competing technology with someform of indirect pressure during material solidification that is widelyused in industry. Gravity or die casting allows articles to solidifyunder little or no pressure. However, articles made by gravity or diecasting methods often suffer from limitations such as porosity in thearticle structure, for example due to high turbulence of the in-gate inhigh pressure die casting, or gas entrapment in gravity casting,resulting in poor mechanical properties. Die casting is also unable toform net-shape articles having high-aspect ratio features withoutdistortion, for example, the round heat sink with high aspect ratio fins10 as shown in FIG. 1 where the height (h) to thickness (t) ratio ofeach fin 10 is greater than 40 to 1, i.e., h:t>40:1. Furthermore,material choice is naturally limited to only casting alloys, whereas foran article to be used as a thermal management component such as a heatsink, it is desirable for it to be formed from low silicon wrought Alalloys or pure Al due to their enhanced thermal dissipating performance.However, these materials cannot be successfully cast due to shrinkagedefects resulting from the casting process.

Squeeze casting or liquid forging forms articles using at leastpartially molten metal under direct pressure in a punch and die set.Improved article tolerances with minimized material distortion andshrinkage defects can be obtained. However, issues of porosity andincomplete part formation due to gas entrapment still remain, especiallyin the formation of articles having high aspect ratio features. This iseven more of a problem when the high aspect ratio features are requiredto be relatively small in size, for example, where the thickness of thefeature is 2 mm or less, as porosities in such small features can makeup a substantial proportion of the feature to result in materialbreakage and/or part failure.

SUMMARY OF THE INVENTION

According to a first exemplary embodiment, there is provided a method offorming a liquid-forged article. The method comprises introducing a meltinto a die cavity; moving a punch relative to the die cavity such thatthe melt enters at least one high aspect ratio cavity in the punch andair is released via at least one air vent insert in the punch; andexerting a forming pressure on the melt while the melt solidifies in thehigh aspect ratio cavity.

The method may further comprise relatively moving the punch away fromthe die cavity while retaining the article in the die cavity. Thearticle is preferably retained in the die cavity by forming a retainingportion of the article within a recess provided in a retaining pindisposed in a die containing the die cavity.

According to a second exemplary aspect, there is provided an apparatusfor forming a liquid-forged article. The apparatus comprises a diecavity for receiving a melt; and a punch configured to exert a formingpressure on the melt, the punch comprising at least one high aspectratio cavity for receiving the melt therein and at least one air ventinsert for allowing air to be released from the high aspect ratiocavity.

For both aspects, air is preferably released through a gas exhaustpassage defined by the air vent insert with the high aspect ratiocavity. Air may further released through at least one gas vent providedin the air vent insert, the gas vent being in fluid communication withthe gas exhaust passage.

The air vent insert preferably comprises a head portion and an insertportion, the head portion being configured for positioning the air ventinsert relative to the high aspect ratio cavity, and the insert portionbeing configured for extending into the high aspect ratio cavity. Theinsert portion may be tapered with a decreasing cross-sectional area asit extends into the high aspect ratio cavity.

The air vent insert may be provided with at least one gas vent, the gasvent being in fluid communication with the gas exhaust passage. The atleast one gas vent is preferably provided in the head portion.

The gas exhaust passage is preferably dimensioned to prevent excessivemelt flow therein for preventing clogging of the gas exhaust passage bysolidified melt. The gas exhaust passage may have a length rangingbetween 2 mm and 6 mm, with a gap tolerance of at least 20 mm betweenthe air vent insert and a wall of the high aspect ratio cavity.

The apparatus may further comprise a retaining pin for retaining thearticle in the die cavity while relatively moving the punch away fromthe die cavity, the retaining pin being disposed in a die containing thedie cavity. The retaining pin preferably comprises a recess forsolidifying the melt therein such to form a retaining portion of thearticle within the recess.

The apparatus may comprise a plurality of high aspect ratio cavities anda corresponding plurality of air vent inserts. The plurality of air ventinserts may be provided individually.

According to a third exemplary aspect, there is provided a liquid-forgedarticle having high aspect ratio features obtained by forming with themethod of the first aspect.

According to a fourth exemplary aspect, there is provided aliquid-forged article having high aspect ratio features obtained usingthe apparatus of the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be fully understood and readily put intopractical effect there shall now be described by way of non-limitativeexample only exemplary embodiments of the present invention, thedescription being with reference to the accompanying illustrativedrawings.

In the drawings:

FIG. 1( a) is a schematic side view illustration of an article havinghigh aspect ratio features, i.e., a round heat sink having tapered fins;

FIG. 1( b) is a schematic top view illustration of the heat sink of FIG.1( a);

FIG. 2 is schematic perspective illustrations of examples of typicalarticles having high aspect ratio features, i.e., heat sinks withtapered fins, pins and radial fins;

FIG. 3 is a flow chart of an exemplary method of forming liquid-forgedarticles;

FIG. 4 is a schematic cross-sectional side view illustration of anexemplary apparatus for forming liquid-forged articles at a forming stepof introducing a melt into a die cavity;

FIG. 5 is the apparatus of FIG. 4 at a forming step of moving a punchinto the die cavity;

FIG. 6 is a schematic cross-sectional side view illustration of air ventinserts in the punch of the apparatus of FIG. 4;

FIG. 7 is a close-up view of an air vent insert of FIG. 6;

FIG. 8 is the apparatus of FIG. 4 at a forming step of relatively movingthe punch away from the die cavity;

FIG. 9 is the apparatus of FIG. 4 at a forming step of ejecting a formedarticle from the die cavity;

FIG. 10 is a schematic side view illustration of removal of a portion ofthe formed article;

FIG. 11 is a schematic side view illustration of high aspect ratiofeatures formed with flash;

FIG. 12 is perspective photographic illustrations of exemplary articlesformed by the exemplary method and apparatus of the present invention;

FIG. 13 (a) is a micrograph of material grain size obtained byconventional casting; and

FIG. 13( b) is a micrograph of material grain size obtained by thepresent invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

As shown in FIGS. 3 to 10, an exemplary method 100 and apparatus 20 areprovided for forming a liquid-forged article having at least one highaspect ratio feature with height to width ratio preferably greater than40:1.

The apparatus 20 comprises a punch 22 having at least one high aspectratio cavity 24 for forming a high aspect ratio feature 34 in aliquid-forged article 30. For articles having a plurality of high aspectratio features 34, a corresponding plurality of high aspect ratiocavities 24 are provided in the punch 22. Each high aspect ratio cavity24 has an internal cavity shape corresponding to a desired feature to beformed in the article 30, such as a pin or a fin as shown in the heatsinks of FIG. 2. The high aspect ratio cavities 24 may be defined bycorresponding high aspect ratio features 25 provided in the punch 22.

The apparatus 20 further comprises a bottom puller or retaining pin 44disposed in a die 42 for part retention and ejection. In an exemplaryembodiment, the bottom puller or retaining pin 44 is provided with atrapezoidally-shaped recess 46 for retaining a formed article using adovetail fit. The punch 22 and die 42 are preferably oriented andprovided as a top punch 22 and a bottom die 42.

As shown in FIG. 4, in a first step of the exemplary method 100, a melt52 comprising an at least partially molten material for forming thearticle 30 is introduced into a die cavity 41 of the die 42, 102, forexample, via a launder 50. The melt temperature may range from 710° C.to 750° C., depending on the material chosen.

Next, as shown in FIG. 5, the punch 22 is moved relative to the diecavity 41 to contact the melt 52 in the die cavity 41, 104, such thatthe melt 52 enters the recess 46 in the retaining pin 44 to form aretaining portion 36 in the liquid-forged article 30. The melt 52 isalso forced to flow and enter the high aspect ratio cavities 24 of thepunch 22. This is preferably achieved by bringing the punch 22 towardsthe die cavity 41 as indicated by the arrow 60. The ram down speed orrelative speed between the punch 22 and the die cavity 41 is preferablyless than 0.5 ms⁻¹ to allow for thorough filling of the cavities 24 withthe melt 52.

Virtually no air bubbles are trapped in or by the melt 52 that entersand fills the high aspect ratio cavities 24 as air is released orallowed to escape via air vent inserts 80 provided in the punch 22, 106.As shown in FIG. 6, each high aspect ratio cavity 24 is provided withits own air vent insert 80 in the punch 22 to allow air to be releasedfrom each high aspect ratio cavity 24. This ensures that all thecavities 24 are fully filled by the melt 52 to virtually eliminateporosity or incomplete part formation in the article 30 when formed.

Each air vent insert 80 preferably has a head portion 82 and an insertportion 84. The head portion 82 and the insert portion 84 may beintegral with each other. The head portion 82 may be accommodated in acorrespondingly shaped recess 23 in the punch 22, and is preferablyconfigured to position the air vent insert 80 relative to itscorresponding high aspect ratio cavity 24. The insert portion 84 isconfigured to extend into the high aspect ratio cavity 24, so that theair vent insert 80 has a generally T-shaped cross-section, as shownhatched in FIG. 7. Accordingly, the air vent insert 80 may comprise, forexample, a disc-shaped head portion 82 with a rod-shaped insert portion84 for forming a high aspect ratio pin, or an elongate, substantiallyrectangular head portion 82 with an elongate, substantially rectangularinsert portion 84 for forming a high aspect ratio fin, depending on thedesired high aspect ratio feature to be formed.

The insert portion 84 is preferably tapered with a decreasingcross-sectional area as it extends into the high aspect ratio cavity 24,such that the insert portion 84 defines a gas exhaust passage 86 withthe high aspect ratio cavity 24. Each air vent insert 80 is preferablyalso provided with at least one gas vent 88 in the head portion 80, thegas vent 88 being in fluid communication with the gas exhaust passage86. The gas exhaust passage 86 and the gas vent 88 allow air to escapefrom the cavity 24 as the melt 52 enters the high aspect ratio cavity 24in the direction shown by the arrow 66 in FIG. 7.

The air vent insert 80 is preferably made of a metal of high heatconductivity, such as a copper-based material, so that as the melt 52comes into contact with the air vent insert 80, the melt 52 is rapidlycooled. Rapid cooling of the melt 52 prevents excessive flow of the melt52 into the gas exhaust passage 86 that can lead to choking or cloggingof the gas exhaust passage 86 or gas vent 88. Accordingly, the gasexhaust passage 86 should be dimensioned to prevent excessive melt flowtherein so as to avoid clogging by solidified melt 52, whilesufficiently allowing air to be released. To that end, the gas exhaustpassage 86 may have a length ranging between 2 mm and 6 mm along thecavity 24, with a gap tolerance of at least 20 μm between the insertportion 84 and a wall 26 of the high aspect ratio cavity 24.

The punch 22 is configured to exert a forming or direct pressure ofbetween 50 MPa and 120 MPa on the melt 52 while the melt 52 solidifiesin the high aspect ratio cavities 24, 108. By exerting a direct formingpressure during solidification, stress is evenly distributed in thesolidified material so that virtually all shrinkage defects areeliminated in the formed article. Releasing air via the air vents 80 andsolidifying the melt 52 under direct pressure of the punch 22 results information of a near net-shape article 30 having high aspect ratiofeatures 34 with virtually no porosity or shrinkage defects.

After the melt 52 has solidified, the punch 22 is moved relative to thedie cavity 41 to separate the liquid-forged article 30 from the punch22. For example, as shown in FIG. 8, the punch 22 may be retracted fromthe die 42 as indicated by the arrow 62, leaving behind theliquid-forged article 30 in the die cavity 41. The punch 22 can beretracted without the formed article 30 sticking to the punch 22 becauseof the dovetailing fit between the recess 46 in the retaining pin 44 andthe retaining portion 36 formed in the article 30. The liquid-forgedarticle 30 is then removed or ejected from the die cavity 41 by relativemovement between the retaining pin 44 and the die 42 until the formedarticle 30 is clear of the die cavity 41. The relative movement maypreferably be achieved by moving the retaining pin 44 into the diecavity 41 in the direction indicated by the arrow 64 in FIG. 9 so thatthe formed article 30 is pushed out of the die cavity 41.

The article 30 may be finished by machining or cutting off the retainingportion 36, as indicated by the dotted line in FIG. 10( a) to result inthe formation of a near net-shape liquid forged article 30 havingfeatures 34 with high aspect ratio as shown in FIG. 10( b). FIG. 11shows close-ups of exemplary high aspect ratio features 70, 72 formedwith flashes 71, 73 as a result of cooling of the melt 52 in the gasexhaust passage 27. The flashes 71 of a thicker feature 70 as shown inFIG. 11( a) may be removed by sand blasting or tumbling. For a thinnerfeature 72 as shown in FIG. 11( b) that may not be rigid enough towithstand the forces of sand blasting or tumbling, the flashes 73 may becut or trimmed away.

Exemplary liquid-forged articles such as heat sinks 90 having highaspect ratio fins 92 or pins 94 formed by the exemplary method 100 andapparatus 20 described above are shown in FIG. 12. As can be seen inFIG. 13 (b), the microstructural grain size of the liquid-forgedarticles is significantly smaller than the grain size of articles formedby conventional casting shown in FIG. 13( a). Smaller grain sizestogether with virtually no porosity result in improved toughness of theliquid-forged articles over conventionally cast articles, therebystrengthening the high-aspect ratio features formed.

Whilst there has been described in the foregoing description exemplaryembodiments of the present invention, it will be understood by thoseskilled in the technology concerned that many variations in details ofdesign, construction and/or operation may be made without departing fromthe present invention. For example, where a plurality of high aspectratio cavities 24 are provided in the punch 22, the correspondingplurality of air vent inserts 80 may be provided individually for easeof air release or may alternatively be provided as an integral unitcomprising a plurality of insert portions 84 disposed on a single headportion 82 for reduced tooling costs. The plurality of high aspect ratiocavities 24 (and their corresponding air vent inserts 80) may or may notbe identical, depending on the desired high aspect ratio features 34 tobe formed. The liquid-forged article 30 may or may not be symmetrical.

1. A method of forming a liquid-forged article, the method comprising:introducing a melt into a die cavity; moving a punch elative to the diecavity such that the melt enters at least one high aspect ratio cavityin the punch and air is released via at least one air vent insert in thepunch; and exerting a forming pressure on the melt while the meltsolidifies in the high aspect ratio cavity.
 2. The method of claim 1,further comprising relatively moving the punch away from the die cavitywhile retaining the article in the die cavity.
 3. The method of claim 2,wherein the article is retained in the die cavity by forming a retainingportion of the article within a recess provided in a retaining pindisposed in a die containing the die cavity.
 4. The method of claim 1,wherein air is released through a gas exhaust passage defined by the airvent insert with the high aspect ratio cavity.
 5. The method of claim 4,wherein air is further released through at least one gas vent providedin the air vent insert, the gas vent being in fluid communication withthe gas exhaust passage.
 6. An apparatus for forming a liquid-forgedarticle, the apparatus comprising: a die cavity for receiving a melt;and a punch configured to exert a forming pressure on the melt, thepunch comprising a least one high aspect ratio cavity for receiving themelt therein and at least one air vent insert for allowing air to bereleased from the high aspect ratio cavity.
 7. The apparatus of claim 6,wherein the air vent insert defines a gas exhaust passage with the highaspect ratio cavity.
 8. The apparatus of claim 6, wherein the air ventinsert comprises a head portion and an insert portion, the head portionbeing configured for positioning the air vent insert relative to thehigh aspect ratio cavity, and the insert portion being configured forextending into the high aspect ratio cavity.
 9. The apparatus of claim8, wherein the insert portion is tapered with a decreasingcross-sectional area as it extends into the high aspect ratio cavity.10. The apparatus of claim 7, the air vent insert defines a gas exhaustpassage with the high aspect ratio cavity, and wherein the air ventinsert is provided with at least one gas vent, the gas vent being influid communication with the gas exhaust passage.
 11. The apparatus ofclaim 10, wherein the air vent insert comprises a head portion and aninsertion e head portion being configured for positioning the air ventinsert relative to the high aspect ratio cavity, and the insert portionbeing configured for extending into the high aspect ratio cavity, andwherein the at least one gas vent is provided in the head portion. 12.The apparatus of claim 7, wherein the gas exhaust passage is dimensionedto prevent excessive melt flow therein for preventing clogging of thegas exhaust passage by solidified melt.
 13. The apparatus of claim 7,wherein the gas exhaust passage has a length ranging between 2 mm and 6mm and wherein there is a gap tolerance of at least 20 mm between theair vent insert and a wall of the high aspect ratio cavity.
 14. Theapparatus of claim 6, further comprising a retaining pin for retainingthe article in the die cavity while relatively moving the punch awayfrom the die cavity, the retaining pin being disposed in a diecontaining the die cavity.
 15. The apparatus of claim 14, wherein theretaining pin comprises a recess for solidifying the melt therein suchto form a retaining portion of the article within the recess.
 16. Theapparatus of claim 6, further comprising a plurality of high aspectratio cavities and a corresponding plurality of air vent inserts. 17.The apparatus of claim 16, wherein the plurality of air vent inserts areprovided individually.
 18. A liquid-forged article having high aspectratio features obtained by forming with a method comprising: introducinga melt into a die cavity; moving a punch relative to the die cavity suchthat the melt enters at least one high aspect ratio cavity in the punchand air is released via at least one air vent insert in the punch; andexerting a forming pressure on the melt while the melt solidifies in thehigh aspect ratio cavity.
 19. A liquid-forged article having high aspectratio features obtained using an apparatus comprising: a die cavity forreceiving a melt; and a punch configured to exert a forming pressure onthe melt, the punch comprising at least one high aspect ratio cavity forreceiving the melt therein and at least one air vent insert for allowingair to he released from the high aspect ratio cavity.